The surface properties of a given material largely determine its interaction with the rest of nature. Most metals and semimetals oxidize spontaneously in air, and the resulting oxide surfaces are highly polar and reactive. They tend to absorb water and volatile organics from the air in a way that is difficult to control outside of a laboratory setting. The result is that properties such as rate of corrosion, surface energy, or adhesion forces can vary greatly from sample to sample and stray far from the ideal for a given application.
One approach to solving this problem is to bond an organic film directly to the oxide surface. This can be performed in a number of ways, and an entire discipline has grown around the area of promoting adhesion to a metal film.
For some applications, greater control over film thickness and morphology is required than can be practiced by bulk technologies. For such systems, the thicknesses and uniformities afforded by conventional approaches fail the minimum specification requirements for the application.
One example of an application that requires strict tolerances for the bonding of organic films to an oxide is stiction control in microelectromechanical systems (MEMS). In MEMS devices, oxidized metal or semimetal surfaces (usually silicon or aluminum oxides) can make contact and adhere through van der Waals, dipole, or capillary forces. Because of the small size of the devices, these forces can often overcome any impulses the systems are capable of producing. However, it is often not practical to coat them directly, because the extreme topography of the devices prevents even coating over the entire device surface.
A currently preferred method for anti-stiction coatings in MEMS devices is the application of a self-limiting monolayer to the active oxide surfaces. Such a coating has the advantage of being of uniform thickness across the device, and thin enough not to perturb the fragile MEMS structures.
Most present solutions to this problem involve the reaction of a discrete silyl ester containing both an organic group directly bound to an activated silicon species, and multiple reactive moieties, to form a monolayer. This solution suffers from several drawbacks, including: the tendency of the organosilicon species to polymerize in solution rather than forming a discrete monolayer; the low vapor pressure of most examples from this class, rendering them difficult to deposit from the vapor phase; the narrow range of commercially available starting materials; and the unknown toxicity of some important members of this class.
Other solutions to this problem include the activation of a surface followed by passivation using organic nucleophiles. Zhu has demonstrated activation of a silicon dioxide surface by chlorine followed by passivation using alcohols. One interesting characteristic of the film created is that it is relatively moisture insensitive, despite the presence of hydrolysable Si—O linkages. It is proposed that the close packing of the organic groups inhibits penetration of water and subsequent hydrolysis of the monolayer. This approach suffers from the need to use corrosive chlorine gas, and is thus ill-suited to manufacturing processes.
Schwartz has approached the problem by depositing a monolayer of zirconium atoms atop of aluminum, followed by passivation of the zirconium monolayer with phenols, carboxylic acids, or phosphonic acids. This method creates a coating that is more thermally robust than carboxylic acids on the native aluminum oxide surface.